A study on the temporally and spatially resolved OH radical distribution of a room-temperature atmospheric-pressure plasma jet by laser-induced fluorescence imaging

Abstract

In this paper, the time and spatially resolved OH distribution of a room-temperature atmospheric-pressure plasma jet is investigated using a laser-induced fluorescence (LIF) method. The plasma jet is generated in room air by applying a nanosecond pulsed high voltage onto a ceramic tube with helium gas flow. It is found that, before the plasma 'bullet' propagates through the region detected by the laser, there are low OH LIF signals, which are from the OH left from previous discharge pulses. After the propagation of the primary plasma 'bullet' (generated by the primary discharge at the rising edge of the voltage pulse) through the detected region, the OH LIF signals are more than doubled. Furthermore, after the propagation of the secondary plasma 'bullet' (generated by the secondary discharge at the falling edge of the voltage pulse) through the detected region, the OH LIF signals are doubled again. Then, after the voltage pulse, the OH LIF signals decay slowly until about 120 µs. Starting from about 120 µs after the voltage pulse, the OH LIF signals have a third increase; its peak value is more than doubled. Detailed investigations find that this is due to the gas flow, which blows the OH generated inside the discharge tube to the detected region. In addition, spatially resolved OH LIF signals show that the signal intensity is stronger on both edges, which gives rise to the donut shape of the OH distribution. Further studies reveal that this might be due to the interaction of the plasma plume with the surrounding water vapor in the air.

Reactive species in non-equilibrium atmospheric-pressure plasmas: Generation, transport, and biological effects

“…This is because the He flow rate affects the surrounding gas diffusion and hence the propagation of the plasma bullet. It has been reported that OH density distribution is at its maximum on the outer surface of the He plasma jet, corresponding to the propagation of the doughnut-shaped plasma bullet [285]. This is different from the results presented in Fig.29 where the OH density has a peak on the center axis of the plasma jet at low He flow rates.…”

“…In [285], the inner diameter of the quartz tube is 2 mm and the He flow rate of 2-4 L/min is used, while in [284] the quartz tube has an inner diameter of 4 mm and the He flow rate is only 1.5 L/min. Thus the He flow velocity at the quartz nozzle in [284] is about 10 times lower compared to [285]. Due to the high flow velocity in the latter case, ambient air could not diffuse into the center of the effluent region.…”

Influence of air diffusion on the OH radicals and atomic O distribution in an atmospheric Ar (bio)plasma jet

“…Similar to reaction (R 21 ), a mechanism has been proposed in [41] for the generation of O: However, because of air diffusion into the jet, the active species map has to follow a 'donut'-like distribution, with higher intensity on the edges, due to the profile of N 2 , H 2 O and O 2 densities in the afterglow. In fact, the 'donut'-like shape of the LIF OH distribution in the afterglow of a He pulsed discharge operated in ambient air has been observed recently in [42]. The appearance of a central dip in the LIF signal after the discharge is also observed by Ono and Oda for a 100 Torr and a 760 Torr pulsed discharge [43].…”

OH Density Measurements by Time-Resolved Broad Band Absorption Spectroscopy in a He-H2O Dielectric Barrier Discharge with Small O2Addition

“…Due to its unique combination of time/space resolution and sensitivity, laser induced fluorescence (LIF) is the technique of choice for application to a wide variety of discharge configurations: pulsed corona discharge, DBD, pulsed DBD, plasma‐jet, pulsed discharge over a liquid surface, pin‐to‐pin single filament discharge, nanosecond H 2 ‐air plasma for plasma assisted combustion at 50–100 Torr . LIF anyway requires a calibration, and absorption spectroscopy can be used to this end or, where high time and space resolution is not required, as an independent technique.…”